Geosynchronous satellites are used, for example, to relay data, voice, telephony and analog/digital television signals from an uplink location to one or many users at various downlink locations. Geosynchronous satellites are placed in orbit in the Earth's equatorial plane at an altitude of 35,784 kilometers. At this altitude, the orbital period of the satellite equals one sidereal day (86,156 seconds). In other words, the satellite orbits the Earth at approximately the same speed as the Earth rotates about it's axis. The result is that the satellite appears stationary to observers on the Earth's surface and can relay communications signals along a direct line-of-sight to all earth-based receivers which are not beyond the horizon of the satellite.
Typically, satellite receiving antennas include a parabolic reflecting dish that focuses the transmitted satellite signal onto an antenna feed. The feed converts the electromagnetic wave energy to electrical signals that can be decoded and/or displayed by the equipment of the end user, for example, a television set.
Because the transmission of communications signals from the satellite is along a direct line-of-sight path, an antenna used to receive such signals from geosynchronous satellites must be pointed accurately toward the satellite in its orbital position. Antennas with large dish apertures have a higher gain than smaller dishes and, therefore, require more accurate aiming at the broadcasting satellite. The dish aperture is the shape of the antenna as viewed down its boresight axis.
The parameters used to describe the direction in which a satellite antenna is pointed are typically azimuth (measured in degrees from the direction of true North at the antenna site) and elevation (measured in degrees from the local horizontal plane). The necessary azimuth ("AZ") and elevation ("EL") to aim an antenna at a particular satellite are easily determined from simple geometric considerations once the latitude and longitude of the antenna, and the longitude of the satellite being targeted are known.
FIG. 1 illustrates a typical satellite dish 101 which is aimed to communicate with a satellite 102 in geosynchronous orbit. As illustrated, the elevation (EL) of the dish is the angle between the radio axis of the dish and the horizontal plane. The azimuth (AZ) of the dish is the angle between the vertical plane containing the radio axis of the dish and true North (N).
Satellite antenna systems typically include the necessary electronic components for monitoring the antenna's azimuth and elevation. Motorized systems capable of moving the antenna can then be placed in a feedback loop with the azimuth and elevation monitoring systems. With such a loop, the antenna can be automatically moved from a known orientation to one in which is it accurately aimed at a target satellite corresponding to a specified azimuth and elevation.
An antenna's azimuth can be roughly measured using a magnetic compass to determine true North. However, because the Earth's magnetic field varies locally at each point on the Earth's surface, the compass reading alone cannot result in a perfectly accurate determination of the antenna's actual azimuth. Therefore, the aim of the satellite will also contain an error factor.
Consequently, there is a need in the art for a method and apparatus of aiming a satellite antenna which more accurately determines the azimuth of the satellite antenna so that the antenna can be more precisely aimed at a target satellite.